Zoom microscope

ABSTRACT

It is an object to provide a zoom microscope of a simple structure which can expand a variable-power range. In order to achieve the object thereof, the zoom microscope includes a replaceable infinity correction objective lens, an aperture stop, an afocal zoom system, and an imaging optical system which are arranged in this order from a specimen side. Further, the aperture stop is disposed on or near a rear focal plane of the objective lens.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application Nos. 2004-344039 and 2004-344086, both filedon Nov. 29, 2004, and No. 2005-193126, filed on Jun. 30, 2005, theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom microscope used for verticalobservation and acquisition of a specimen image.

2. Description of the Related Art

A known zoom microscope is structured by combining a zoom imaging lenswith an infinity correction objective lens (see, for example, JapaneseUnexamined Patent Application Publication No. Hei 6-18784). In thisstructure, observation magnification of a specimen image is arbitrarilyvariable by moving a variable-power lens group in the zoom imaging lensin an optical axis direction.

Further, it has been proposed that a zoom microscope is structured byattaching an infinity correction zoom objective lens to a revolver of atypical microscope, thereby combining the zoom objective lens and animaging lens of the microscope (see, for example, Japanese UnexaminedPatent Application Publication No. 2004-133341). In such a structure,observation magnification of a specimen image is arbitrarily variable bymoving a variable-power lens group in the zoom objective lens in anoptical axis direction.

However, intended variable-power ranges of both of the aforesaid zoommicroscopes are narrow, and they do not include a low variable power,for example, (about 0.5 to about 2). The expansion of the variable-powerrange (for example, to the low-power side) is desirable, however, it isdifficult for the zoom microscope of the former document to set theposition of an entrance pupil of the objective lens at infinity over theentire variable-power range since a pupil of the objective lens is inthe zoom imaging lens and the position of the pupil of the zoom imaginglens moves due to the power variation. On the other hand, in the zoommicroscope of the latter document a pupil is located between anobjective lens portion and a zoom portion (including the variable-powerlens group) in the zoom objective lens, so that it is possible toprovide a plurality of zoom objective lenses of different variable-powerranges, and expand the variable-power range by selectively attachingthem to the revolver. However, there is a disadvantage that theprovision of the zoom portion for each zoom objective lens results inincreasing the scale of the device substantially.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a zoom microscope ofa simple structure which can expand a variable-power range.

A zoom microscope of the present invention includes a replaceableinfinity correction objective lens, an aperture stop, an afocal zoomsystem, and an imaging optical system which are arranged in this order,seen from a specimen side. The aperture stop is disposed on or near arear focal plane of the objective lens.

Preferably, the afocal zoom system has a variable-power lens groupmovable in an optical axis direction, and the aperture stop has a stopdiameter that is variable in accordance with the movement of thevariable-power lens group.

Preferably, the zoom microscope further includes a coaxialepi-illuminator provided between the afocal zoom system and the imagingoptical system to form an image of a light source on or near theaperture stop.

Preferably, the zoom microscope further includes a holding memberreplaceably holding the objective lens, and the aperture stop isdisposed on an image side of the holding member.

Preferably, a focal length fL of the afocal zoom system when combinedwith the imaging optical system, and a maximum image height Ymax in thelower-power end state satisfy a following conditional expression.0.05<Ymax/fL<0.16

Preferably, the zoom microscope further includes an optical member forphase contrast observation that is removably inserted between theobjective lens and the afocal zoom system.

Preferably, the zoom microscope further includes a fluorescenceepi-illuminator provided between the objective lens and the afocal zoomsystem.

Preferably, the objective lens is plural in number and the pluralobjective lenses are different from each other in magnification, anddistances from abutting joint surfaces of the plural objective lenses tothe rear focal plane are substantially same.

Preferably, the optical member for phase contrast observation is commonto the plural objective lenses.

Preferably, the optical member for phase contrast observation is abirefringence optical member for differential interference contrastobservation.

Another zoom microscope of the present invention includes: a replaceableinfinity correction objective lens; an optical member for phase contrastobservation removably inserted on or near a rear focal plane of theobjective lens; an afocal zoom system; an aperture stop disposed on ornear an entrance pupil plane of the afocal zoom system; and an imagingoptical system disposed on an image side of the afocal zoom system.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature, principle, and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by identical reference numbers, in which:

FIG. 1 is a view showing a whole configuration of a zoom microscope 10of a first embodiment;

FIG. 2 is a view to illustrate the replacement of an objective lens 11;

FIG. 3( a) and FIG. 3( b) are views to illustrate a change in stopdiameter of an aperture stop 12, comparing a low-power state (a) and ahigh-power state (b) of an afocal zoom system 13;

FIG. 4 is a view showing an example of a concrete configuration of theobjective lens 11;

FIG. 5 is a view showing an example of a concrete configuration of theafocal zoom system 13;

FIG. 6 is a view showing an example of a concrete configuration of animaging optical system 14;

FIG. 7 is a view showing various aberrations when the zoom microscope 10is in a low-power end state;

FIG. 8 is a view showing various aberrations when the zoom microscope 10is in a mid-power state;

FIG. 9 is a view showing various aberrations when the zoom microscope 10is in a high-power end state;

FIG. 10 is a view showing a whole configuration of a zoom microscope 40of a second embodiment;

FIG. 11 is a view showing a whole configuration of a zoom microscope 50of a third embodiment;

FIG. 12 is a view showing an example of a concrete configuration of anafocal zoom system 70 of a fourth embodiment;

FIG. 13 is a view showing various aberrations when a zoom microscopeincluding the afocal zoom system 70 is in a low-power end state;

FIG. 14 is a view showing various aberrations when the zoom microscopeincluding the afocal zoom system 70 is in a mid-power state;

FIG. 15 is a view showing various aberrations when the zoom microscopeincluding the afocal zoom system 70 is in a high-power end state;

FIG. 16 is a view showing a whole configuration of a zoom microscope 10of a fifth embodiment;

FIG. 17 is a view to illustrate the replacement of an objective lens 11;

FIG. 18( a) and FIG. 18( b) are views to illustrate a change in stopdiameter of an aperture stop 12, comparing a high-power state (a) and alow-power state (b) of an afocal zoom system 13;

FIG. 19 is a view to illustrate a DIC prism 21 and an analyzer 22 fordifferential interference contrast observation of a specimen 10A;

FIG. 20 is a view showing a whole configuration of a zoom microscope 40of a sixth embodiment;

FIG. 21 is a view showing a whole configuration of a zoom microscope 50of a seventh embodiment;

FIG. 22 is a configuration view (optical path view) of an objective lensof an eighth embodiment;

FIG. 23 is a view showing various aberrations in the objective lens ofthe eighth embodiment when focal length of a subsequent zoom opticalsystem is 100 mm;

FIG. 24 is a view showing various aberrations in the objective lens ofthe eighth embodiment when the focal length of the subsequent zoomoptical system is 400 mm;

FIG. 25 is a view showing various aberrations in the objective lens ofthe eighth embodiment when the focal length of the subsequent zoomoptical system is 750 mm;

FIG. 26 is a configuration view (optical path view) of an objective lensof a ninth embodiment;

FIG. 27 is a view showing various aberrations in the objective lens ofthe ninth embodiment when focal length of a subsequent zoom opticalsystem is 100 mm;

FIG. 28 is a view showing various aberrations in the objective lens ofthe ninth embodiment when the focal length of the subsequent zoomoptical system is 400 mm;

FIG. 29 is a view showing various aberrations in the objective lens ofthe ninth embodiment when the focal length of the subsequent zoomoptical system is 750 mm;

FIG. 30 is a configuration view (optical path view) of an objective lensof a tenth embodiment;

FIG. 31 is a view showing various aberrations in the objective lens ofthe tenth embodiment when focal length of a subsequent zoom opticalsystem is 100 mm;

FIG. 32 is a view showing various aberrations in the objective lens ofthe tenth embodiment when the focal length of the subsequent zoomoptical system is 400 mm;

FIG. 33 is a view showing various aberrations in the objective lens ofthe tenth embodiment when the focal length of the subsequent zoomoptical system is 750 mm;

FIG. 34 is a configuration view (optical path view) of an objective lensof an eleventh embodiment;

FIG. 35 is a view showing various aberrations in the objective lens ofthe eleventh embodiment when focal length of a subsequent zoom opticalsystem is 100 mm;

FIG. 36 is a view showing various aberrations in the objective lens ofthe eleventh embodiment when the focal length of the subsequent zoomoptical system is 400 mm;

FIG. 37 is a view showing various aberrations in the objective lens ofthe eleventh embodiment when the focal length of the subsequent zoomoptical system is 750 mm; and

FIG. 38 is a view showing the objective lens attached to a turret.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings.

First Embodiment

A zoom microscope 10 of a first embodiment has an objective lens 11, anaperture stop 12, an afocal zoom system 13, and an imaging opticalsystem 14, which are arranged in this order from a specimen 10A side asshown in FIG. 1. Ray bundles emitted from respective points of thespecimen 10A are turned to parallel ray bundles via the objective lens11, are variably magnified via the afocal zoom system 13, and arecollected via the imaging optical system 14 to reach an image plane 10B.

In order to observe an image of the specimen 10A formed on the imageplane 10B, an image sensor, for example, a CCD or the like is disposedon the image plane 10B. Alternatively, instead of the imaging opticalsystem 14, an observation binocular tube (eyepiece tube), a phototube, atrinocular tube for observation and photographing, or the like whichincludes an equivalent imaging optical system can be disposed accordingto an intended use. The use of the zoom microscope 10 of the firstembodiment enables vertical observation and image acquisition of thespecimen 10A.

Further, in the zoom microscope 10 of the first embodiment, the afocalzoom system 13 is composed of a first lens group G1 with a positiverefractive power, a second lens group G2 with a negative refractivepower, a third lens group G3 with a positive refractive power, and afourth lens group G4 with a weak positive refractive power, which arearranged in this order from the specimen 10A side. The second lens groupG2 and the third lens group G3 are variable-power groups. Therefore, bymoving the variable-power lens groups (G2, G3) in an optical axisdirection while fixing the first lens group G1 and the fourth lens groupG4, it is possible to arbitrarily vary magnification for observing theimage of the specimen 10A. The observation magnification is determinedby a ratio of focal length of the objective lens 11 and focal length ofthe afocal zoom system 13 (combined with the imaging optical system 14).

Further, the objective lens 11 is of an infinity correction type, and arear focal plane of the objective lens 11 is located on an image side ofits lens plane nearest to the image side (between the objective lens 11and the afocal zoom system 13). The aperture stop 12 is disposed on therear focal plane (or near the rear focal plane) of the objective lens11. Therefore, the position of an object-side entrance pupil of theobjective lens 11 is infinite (telecentric), and principal rays of theray bundles emitted from the respective points of the specimen 10A enterthe objective lens 11 in parallel to the optical axis direction.

Further, setting the position of the aperture stop 12 as the position ofan entrance pupil of the afocal zoom system 13 also makes it possible toset the position of the entrance pupil of the objective lens 11 atinfinity in the entire variable-power range even when the aforesaidvariable-power lens groups (G2, G3) are moved for power variation. Inshort, irrespective of a variable power by the afocal zoom system 13,telecentricity on the object side of the objective lens 11 can bemaintained.

Moreover, in the zoom microscope 10 of the first embodiment, theobjective lens 11 is replaceably attached to a slider or turret(revolver) as shown in FIG. 38. A turret rotation portion is rotatablyattached to a turret fixation portion. Specifically, plural kinds of theobjective lenses 11 (for example, a low-power objective lens 11(1), ahigh-power objective lens 11(2) shown in FIGS. 2 and 38, and so on) areattached to the turret, and the kind of the objective lens 11 ischangeable by the rotation (sliding) of the turret.

Further, in the zoom microscope 10 of the first embodiment, thedistances from abutting joint surfaces (surfaces attached to the turret)of the respective objective lenses 11 to the rear focal plane are equal.Therefore, even when the objective lens 11 is replaced, it is possibleto coincide the rear focal plane of the objective lens 11 (or thevicinity thereof) and an arrangement plane of the aperture stop 12 witheach other while the aperture stop 12 is kept fixed. It is also possibleto maintain the state in which the arrangement plane of the aperturestop 12 and the position of the entrance pupil of the afocal zoom system13 coincide with each other.

Therefore, even when the objective lens 11 is replaced, telecentricityon the object side of the objective lens 11 can be maintainedirrespective of the variable-power state (i.e., the positions of thevariable-power lens groups G2, G3) by the afocal zoom system 13.

When the low-power objective lens 11(1) is disposed on the optical axisof the afocal zoom system 13, it is possible to change the magnificationfor observing the image of the specimen 10A according to a ratio offocal length of the objective lens 11(1) and focal length of the afocalzoom system 13 (combined with the imaging optical system 14) whilemaintaining telecentricity on the object side of the objective lens11(1). Likewise, when the high-power objective lens 11(2) is disposed onthe optical axis of the afocal zoom system 13, it is possible to changethe magnification for observing the image of the specimen 10A accordingto a ratio of focal length of the objective lens 11(2) and the focallength of the afocal zoom system 13 (combined with the imaging opticalsystem 14), while maintaining telecentricity on the object side of theobjective lens 11(2).

A range of the observation magnification (variable-power range) will bedescribed assuming that, for example, the focal length of the low-power(1×) objective lens 11(1) is 100 mm, the focal length of the high-power(4×) objective lens 11(2) is 25 mm, focal length of a very low-power(0.5×) objective lens (not shown) is 200 mm, and the focal length of theafocal zoom system 13 (combined with the imaging optical system 14) is100 mm to 750 mm. When the very low-power objective lens is used, thevariable-power range is 0.5 to 3.75. When the lower-power objective lens11(1) is used, the variable-power range is 1 to 7.5. When the high-powerobjective lens 11(2) is used, the variable-power range is 4 to 30. Theoverall variable-power range is 0.5 to 30.

Thus, in the zoom microscope 10 of the first embodiment, the replaceableobjective lenses 11 (see the objective lenses 11(1), 11(2) in FIG. 2)share the afocal zoom system 13, and the variable-power range is shiftedby the replacement of the objective lenses 11, so that it is possible toexpand the variable-power range with a simple structure (i.e., only oneafocal zoom system 13).

The use of the very low-power objective lens (for example, 0.5×objective lens) as one of the replaceable objective lenses 11 makes itpossible to expand the variable-power range up to a very lowvariable-power range (about 0.5 to about 2) with a simple structure. Inthis case, the zoom microscope 10 functions as a “macrozoom microscope”,so that macro observation of the specimen 10A also is made possible. Themacro observation refers to observing a relatively large specimen 10Asuch as, for example, a metal specimen and a mechanical part (such as agear). In order to respond to a change in thickness of the specimen 10A,the whole observation optical system from the objective lens 11 to theimaging optical system 14 is vertically moved.

Further, in the zoom microscope 10 of the first embodiment, it ispossible to ensure telecentricity on the object side irrespective of thevariable-power state of the afocal zoom system 13 (i.e., the positionsof the variable-power lens groups (G2, G3)) even when the objective lens11 is replaced, as has been described above. Therefore, in the macroobservation of the specimen 10A selectively using the low-powerobjective lens while moving the variable-power lens groups (G2, G3), itis similarly possible to ensure telecentricity on the object side.

Further, in the zoom microscope 10 of the first embodiment,telecentricity on the object side of the objective lens 11 can beensured in the whole wide variable-power range (for example, a powerrange from 0.5 to 30), which enables coaxial epi-illumination free ofvignetting.

Further, in the zoom microscope 10 of the first embodiment, it ispossible to realize a diversity of observation methods in a widevariable-power range (also in a low variable-power range) by inserting acoaxial epi-illuminator, a fluorescence epi-illuminator, a phototube, orthe like in front of or at the back of the afocal zoom system 13 (i.e.,between the objective lens 1 and the afocal zoom system 13, or betweenthe afocal zoom system 13 and the imaging optical system 14). Note thatwhen the specimen 10A to be observed is transparent, a transilluminatoris disposed under the specimen 10A (opposite the objective lens 11).

Further, in the zoom microscope 10 of the first embodiment, inserting anoptical member for phase contrast observation between the objective lens11 and the aperture stop 12 enables phase contrast observation (forexample, differential interference contrast observation or the like) ofthe specimen 10A with varied low powers. Such a configuration will beconcretely described later in a fifth embodiment to a seventhembodiment.

Further, in the zoom microscope 10 of the first embodiment, it ispreferable to use a variable aperture stop as the aperture stop 12,thereby making stop diameter thereof variable according to the movementof the variable-power lens groups (G2, G3) of the afocal zoom system 13(see FIG. 3( a) and FIG. 3( b)). In FIG. 3( a) and FIG. 3( b), among theray bundles emitted from the respective points of the specimen 10A, acenter ray bundle is shown by the broken lines, and peripheral principalrays nearest to the image are shown by the two-dot chain lines. Notethat not only the principal rays shown in the drawing but also not-shownprincipal rays are parallel to the optical axis direction, so thattelecentricity on the object side of the objective lens 11 is ensured.

FIG. 3( a) shows a state where the lens groups (G2, G3) are moved to alow-power side. By reducing the stop diameter of the aperture stop 12 inconjunction with this movement, it is possible to restrict an openingangle of the center ray bundle to a small value. In this case, theobservation of a deep focal depth and a wide field of view is possiblewith low NA. FIG. 3( b) shows a state where the lens groups (G2, G3) aremoved to a high-power side. By increasing the stop diameter of theaperture stop 12 in conjunction with this movement, it is possible towiden the opening angle of the center ray bundle. In this case,high-resolution observation with high NA is possible.

Further, in the zoom microscope 10 of the first embodiment, it ispreferable that the second lens group G2 of the afocal zoom system 13satisfies the following conditional expression (1). The conditionalexpression (1) shows a desirable range of a magnification βL2 of thesecond lens group G2 in a low-power end state (see, for example, FIG. 3(a)).−0.1<β2L<−0.3  (1)

β2L under the lower limit value of the conditional expression (1) is notdesirable since it results in a large movement amount of the second lensgroup G2, which leads to increase and complication of a mechanism formoving the variable-power lens groups (G2, G3). In order to reduce themovement amount of the second lens group G2 under the same condition,the refractive power of the second lens group G2 has to be made high,which makes difficult aberration correction in a screen peripheralportion. On the other hand, β2L over the upper limit value results in alarge interval between the second lens group G2 and the third lens groupG3 on the lower-power side. Accordingly, entrance height of peripheralray bundles entering the third group G3 is high, which leads to sizeincrease of the third lens group G3 and thus is not desirable.Therefore, by satisfying the conditional expression (1), it is possibleto downsize the afocal zoom system 13, in particular, to set themovement amount of the second lens group G2 to an appropriate value,thereby achieving downsizing of the third lens group G3.

Further, in the zoom microscope 10 of the first embodiment, it ispreferable that the third lens group G3 of the afocal zoom system 13satisfies the following conditional expression (2). The conditionalexpression (2) shows a desirable range of a magnification β3L of thethird lens group G3 in the low-power end state (see, for example, FIG.3( a)).−0.01<1/β3L<−0.04  (2)

1/β3L below the lower limit value of the conditional expression (2)results in a high refractive power of the third lens group G3, whichmakes difficult aberration correction in the screen peripheral portionon the low-power side. On the other hand, 1/β3L over the upper limitvalue of the conditional expression (2) is not desirable since itresults in a low refractive power of the third lens group G3 and thusthe entrance height of the peripheral ray bundles entering the fourthlens group G4 is high, leading to size increase of the fourth lens groupG4. Therefore, satisfying the conditional expression (2) makes itpossible to downsize the afocal zoom system 13, in particular, toachieve downsizing of the fourth lens group G4 and good opticalperformance in the screen peripheral portion on the low-power side.

Further, in the zoom microscope 10 of the first embodiment, the focallength of the afocal zoom system 13 (combined with the imaging opticalsystem 14) and the maximum image height preferably satisfy the followingconditional expression (3). The conditional expression (3) shows adesirable range of a ratio of the focal length fL of the afocal zoomsystem 13 (combined with the imaging optical system 14) and the maximumimage height Ymax, in the lower-power end state.0.05<Ymax/fL<0.16  (3)

The ratio below the lower limit value of the conditional expression (3)results in a long focal length of the afocal zoom system 13 (combinedwith the imaging optical system 14) relative to the maximum imageheight, in the low-power end state. This is not desirable because thisin turn increases the focal length of the very low-power (0.5×)objective lens, leading to size increase of a zoom microscope systemwhen it is used for very low magnification observation. On the otherhand, the ratio over the upper limit value of the conditional expression(3) is not desirable since it results in a large angle at which theperipheral ray bundles closest to the image enter the afocal zoom system13 on the low-power side, leading to size increase of the third lensgroup G3 and the fourth lens group G4, and in addition, leading todifficulty in aberration correction of the screen peripheral portion.Therefore, by satisfying the conditional expression (3), it is possibleto downsize the zoom microscope system used for the verylow-magnification observation and to achieve good optical performance inthe screen peripheral portion on the low-power side.

Next, concrete configurations of the objective lens 11, the afocal zoomsystem 13, and the imaging optical system 14 in the zoom microscope 10of the above-described first embodiment will be described.

As shown in FIG. 4, in the objective lens 11, a flat plate glass 21, abiconvex lens 22, a positive meniscus lens 23 having a convex surface onthe object side, a cemented lens of a biconvex lens 24 and a biconcavelens 25, a cemented lens of a biconcave lens 26 and a biconvex lens 27,and a biconvex lens 28 are arranged in this order from the object side(the left side of the drawing). The magnification of the objective lens11 is 1×. When the coaxial epi-illumination is assumed, a quarterwavelength plate is used as the flat plate glass 21. In a case where thecoaxial epi-illumination is not assumed, a dummy glass (protectiveglass) is preferably used as the flat plate glass 21.

Table 1 shows examples of various initial values of the objective lens11 shown in FIG. 4.

TABLE 1 f = 100 NO. r d ν n 0 36.0131 1.000000 1 0.0000 2.2500 64.141.516330 2 0.0000 0.5000 1.000000 3 44.9980 5.5000 68.33 1.592400 4−154.7100 0.5000 1.000000 5 25.2310 5.0000 68.33 1.592400 6 53.22206.5000 1.000000 7 25.4730 7.5000 82.52 1.497820 8 −40.2700 3.0000 52.321.754998 9 11.2760 15.2500 1.000000 10 −12.8730 6.0000 46.57 1.804000 1159.7860 5.5000 82.52 1.497820 12 −18.8950 1.0000 1.000000 13 1128.31005.5000 68.33 1.592400 14 −21.6003 10.0000 1.000000 15 SP 1.000000

In Table 1, the surface number 0 corresponds to an object plane, thesurface numbers 1 to 14 are numbers assigned to lens surfaces in orderfrom the object side, and the surface number 15 corresponds to theaperture stop 12. Minus (−) of the radius of curvature (r) of each lenssurface indicates that the lens surface is convex toward the image side.The plane interval (d) is lens thickness or air space on the opticalaxis. As for the others, ν is Abbe number for the d-line (587 nm) and nis a refractive index for the d-line. The working distance docorresponds to an interval (=36.0131) from the object plane (0) to thelens surface (1) closest to the object side. The distance between thelens surface (12) closest to the image side and the aperture stop 12 is10 mm. f represents the focal length of the objective lens 11.

Concrete examples of such an objective lens 11 will be further describedlater in an eighth embodiment to an eleventh embodiment. The objectivelenses described in the eighth embodiment to the eleventh embodiment areinfinity objective lenses with pupils protruding toward the image sidein the above-described structure.

Further, the afocal zoom system 13 is composed of the above-describedfour lens groups G1, G2, G3, G4 as shown in FIG. 5 (see FIG. 3( b) aswell). The first lens group G1 is composed of: a cemented lens of anegative meniscus lens 31 having a concave surface on the object sideand a biconvex lens 32; and a positive meniscus lens 33 having a convexsurface on the object side. The second lens group G2 is composed of: abiconcave lens 34, a cemented lens of a positive meniscus lens 35 havinga concave surface on object side and a biconcave lens 36; and a negativemeniscus lens 37 having a concave surface on the object side. The thirdlens group G3 is composed of: a positive meniscus lens 38 having aconcave surface on the object side; and a cemented lens of a biconvexlens 39 and a negative meniscus lens 40 having a concave surface on theobject side. The fourth lens group G4 is composed of a cemented lens ofa positive meniscus lens 41 having a concave surface on the object sideand a negative meniscus lens 42 having a concave surface on the objectside.

In the afocal zoom system 13 in FIG. 5, for power variation from thelow-power end to the high-power end, the second lens group G2 is movedtoward the image side while the first lens group G1 and the fourth lensgroup G4 are fixed, and the third lens group G3 is moved in a directionfor correcting a focus shift ascribable to the movement of the secondlens group G2 (see FIG. 3( a)→FIG. 3( b)). It is preferable to increasethe stop diameter of the aperture stop 12 in conjunction with the powervariation from the lower-power end to the high-power end of the afocalzoom system 13.

Table 2 shows examples of various original values of the afocal zoomsystem 13 and the aperture stop 12 shown in FIG. 5.

TABLE 2 cf = 100~750 Fno = 16.7~37.5 NO. r d ν n d0′  1 SP 15.00001.000000  2 120.1967 2.0000 39.57 1.804400  3 48.7980 3.0000 82.561.497820  4 −509.0866 0.5000 1.000000  5 50.4610 3.0000 82.56 1.497820 6 3179.8129 d1 1.000000  7 −108.0082 1.5000 35.71 1.902650  8 25.81942.0000 1.000000  9 32.8474 3.5000 23.78 1.846660 10 −19.0003 1.000060.29 1.620410 11 31.8448 1.5000 1.000000 12 −25.9839 1.5000 35.711.902650 13 228.2515 d2 1.000000 14 838.2380 6.0000 82.56 1.497820 15−31.9728 0.2000 1.000000 16 136.9685 6.0000 82.56 1.497820 17 −39.21202.0000 28.55 1.795040 18 −92.0449 d3 1.000000 19 −339.8016 5.5000 36.241.620040 20 −40.8020 1.5000 39.57 1.804400 21 −124.4210 8.5017 1.00000022 PL 0.0000 1.000000 Fai 6.0000 12.0000 20.0000 f 100.0000 250.0000750.0000 D0 0.0000 0.0000 0.0000 d1 2.6024 42.4730 60.3676 d2 22.837218.1594 2.5051 d3 43.3588 8.1659 5.9257

In Table 2, the surface number 1 corresponds to the aperture stop 12,the surface numbers 2 to 21 are numbers assigned to lens surfaces inorder from the object side, and the surface number 22 corresponds to alens-tube abutting joint surface. The distance between the lens surface(2) closest to the object side and the aperture stop 12 is 15 mm. frepresents the focal length of the whole lens system when the distancefrom the object plane to the aperture stop 12 is expressed as do′=∞. Fnorepresents F number and fai represents the stop diameter of the aperturestop 12.

Values corresponding to the aforesaid conditional expressions (1), (2)are as follows.β2L=−0.217  (1)1/β3L=0.008  (2)Ymax/fL=0.110  (3)

The imaging optical system 14 is composed of four lenses 61 to 64 asshown in FIG. 6. Table 3 shows examples of various original values ofthe imaging optical system 14 shown in FIG. 6. In Table 3, the surfacenumber 1 corresponds to a lens-tube abutting joint surface (PL), thesurface numbers 2 to 7 are numbers assigned to lens surfaces in orderfrom the object side. The distance between the surface closest to theobject side and the lens-tube abutting joint surface (PL) is 1 mm. frepresents the focal length of the imaging optical system 14 and Bfrepresents the distance between the plane closest to the image side andthe image plane.

TABLE 3 f = 250 Bf = 186.0354 NO. r d ν n 1 PL 1.0000 1.000000 2 63.34255.5000 68.33 1.592400 3 −156.8517 2.0000 28.56 1.795040 4 712.235620.0000 1.000000 5 46.5880 4.5000 37.90 1.723421 6 708.2065 2.0000 49.451.772789 7 36.4262 Bf 1.000000

FIG. 7 to FIG. 9 show various aberrations (spherical aberration,astigmatism, distortion aberration, chromatic aberration ofmagnification, and coma aberration) regarding the zoom microscope 10including the focal zoom system 13 and the aperture stop 12 based on thevarious original values in Table 2 and the imaging optical system 14based on the various original values in Table 3. FIG. 7 corresponds tothe low-power end state (f =100), FIG. 8 to the mid-power state (f=250),and FIG. 9 to the high-power end state (f=750). In FIG. 7 to FIG. 9, Fnorepresents F number, Y represents image height, d represents d-line(λ=587 nm), and g represents the g-line (λ=436 nm). As is apparent fromthe drawings, various aberrations are well corrected in the zoommicroscope 10 of the first embodiment.

Second Embodiment

A zoom microscope 40 of a second embodiment is configured such that acoaxial epi-illuminator (43 to 46) is provided between the afocal zoomsystem 13 and the imaging optical system 14 of the zoom microscope 10(FIG. 1) of the first embodiment, as shown in FIG. 10. The zoommicroscope 40 is used for vertical observation (image acquisition) of atransparent specimen 10A in industrial application.

In the coaxial epi-illuminator (43 to 46), an outgoing ray bundle from afiber light source 43 is led to the afocal zoom system 13 via acollector lens 44, a relay lens 45, and a beam splitter 46 to reach theaperture stop 12 via the afocal zoom system 13. At this time, an imageof a light source (end-face image of the fiber light source 43) isformed on the aperture stop 12 (or near the aperture stop 12) by thecoaxial epi-illuminator (43 to 46).

Thereafter, the ray bundle that has passed through the aperture stop 12enters the specimen 10A via the objective lens 11. A principal ray ofthe ray bundle directed toward the specimen 10A from the objective lens11 becomes parallel to an optical axis direction, since, as describedabove, the rear focal plane of the objective lens 11 is located near theaperture stop 12 and telecentricity on the object side of the objectivelens 11 can be ensured in the whole wide variable-power range (forexample, a power range from 0.5 to 30). In short, illumination for thespecimen 10A is coaxial epi-illumination (so-called telecentricillumination) free of vignetting.

This allows good bright-field observation of the transparent specimen10A. In particular, when macro observation in a low-power range (about0.5 to about 2) is carried out, poor telecentricity on the object sideis not desirable since it causes the principal ray in a screenperipheral portion (ray passing through the center of a pupil) to passthrough a pupil plane at a larger angle, which causes vignetting in theillumination in a field of view. In the zoom microscope 40 of thisembodiment, telecentricity on the object side can be ensured even in thelow-power range, which enables good macro observation by the coaxialepi-illumination free of vignetting.

Further, in the zoom microscope 40 of this embodiment, the coaxialepi-illuminator (43 to 46) is provided between the afocal zoom system 13and the imaging optical system 14 to illuminate the specimen 10A via theafocal zoom system 13 (in other words, the afocal zoom system 13 isshared by an illumination system and an observation system), so that itis possible to change an illuminated range in conjunction with a changein an observation range of the specimen 10A at the time of the powervariation. Therefore, efficient coaxial epi-illumination is achieved.

Third Embodiment

A zoom microscope 50 of a third embodiment is configured such that afluorescence epi-illuminator (51 to 56) is provided between theobjective lens 11 and the aperture stop 12 of the zoom microscope 10(FIG. 1) of the first embodiment, as shown in FIG. 11. The zoommicroscope 50 is used for vertical observation (image acquisition) basedon feeble light from a specimen 10A such as a biospecimen marked by afluorescent substance.

In the fluorescence epi-illuminator (51 to 56), an outgoing ray bundlefrom a fiber light source 51 enters an excitation filter 54 via acollector lens 52, a relay lens 53, and a not-shown aperture stop. Theexcitation filter 54 transmits only a ray bundle (excitation light) in awavelength band necessary for exciting the specimen 10A. The excitationlight from the excitation filter 54 is led to the objective lens 11 viaa dichroic mirror 55 to enter the specimen 10A via the objective lens11.

Fluorescence emitted from the specimen 10A enters the aperture stop 12via the objective lens 11, the dichroic mirror 55, and a barrier filter56 and thereafter reaches an image plane 10B via the aperture stop 12,the afocal zoom system 13, and the imaging optical system 14. Thefluorescence from the specimen 10A is feeble, and enters the dichroicmirror 55 together with unnecessary excitation light reflected on thespecimen 10A, but the unnecessary excitation light is cut off when itpasses through the dichroic mirror 55 and the barrier filter 56, so thatonly the feeble fluorescence can be led to the image plane 10B. Thisenables fluorescence observation of the specimen 10A in the whole widevariable-power range (for example, a power range from 0.5 to 30).

Further, in the zoom microscope 50 of this embodiment, the excitationlight from the fluorescence epi-illuminator (51 to 56) passes onlythrough the objective lens 11 in the observation optical system (fromthe objective lens 11 to the imaging optical system 14) and passesthrough neither of the afocal zoom system 13 and the imaging opticalsystem 14. This can minimize self-fluorescence generated in each lenselement of the observation optical system by the excitation light. As aresult, fluorescence observation with good contrast is enabled.

Further, the fluorescence epi-illuminator (51 to 56) may be a zoomoptical system. In this case, it is possible to change an illuminationrange of the excitation light and NA of illumination, which enablesefficient fluorescence illumination appropriate for the variable-powerrange of the observation optical system. Moreover, mechanically linkinga variable-power mechanism of the zoom optical system of the aforesaidepi-illuminator with a variable-power mechanism of the afocal zoomsystem 13 enables efficient fluorescence illumination appropriate forthe variable-power range of the observation optical system by a singlepower-variation operation.

Note that in the zoom microscope 50 of this embodiment, telecentricityof the illumination is not necessary since the fluorescence emitted fromthe fluorescent substance in the specimen 10A irradiated with theexcitation light is observed.

Fourth Embodiment

Here, an afocal zoom system 70 with a three-group structure will bedescribed.

The afocal zoom system 70 is composed of three lens groups G1, G2, G3 asshown in FIG. 12. Further, the first lens group G1 is composed of: acemented lens of a negative meniscus lens 71 having a concave surface onan object side and a biconvex lens 72; and a positive meniscus lens 73having a convex surface on the object side. The second lens group G2 iscomposed of: a biconcave lens 74; a cemented lens of a positive meniscuslens 75 having a concave surface on the object side and a negativemeniscus lens 76 having a concave surface on the object side; and anegative meniscus lens 77 having a concave surface on the object side.The third lens group G3 is composed of: a positive meniscus lens 78having a concave surface on the object side; and a cemented lens of abiconvex lens 79 and a negative meniscus lens 80 having a concavesurface on the object side.

In the afocal zoom system 70 in FIG. 12, for power variation from alow-power end to a high-power end, the second lens group G2 is moved toan image side while the first lens group G1 is fixed, and the third lensgroup G3 is moved in a direction for correcting a focus shift ascribableto the movement of the second lens group G2. It is preferable toincrease stop diameter of an aperture stop 12 in conjunction with thepower variation from the low-power end to the high-power end of theafocal zoom system 70.

Table 4 shows examples of various original values of the afocal zoomsystem 70 and the aperture stop 12 shown in FIG. 12.

TABLE 4 f = 100~750 Fno = 16.7~37.5 NO. r d ν n d0  1 SP 5.0000 1.000000 2 76.4790 2.0000 39.57 1.804400  3 39.5970 4.0000 82.56 1.497820  4−122.2643 0.5000 1.000000  5 34.9517 2.5000 82.56 1.497820  6 49.3068 d11.000000  7 −88.4247 1.5000 35.71 1.902650  8 34.7688 2.0000 1.000000  9−61.3206 3.5000 23.78 1.846660 10 −9.8577 1.0000 60.14 1.620409 11−35.4310 1.0000 1.000000 12 −14.7762 1.5000 35.71 1.902650 13 −225.5269d2 1.000000 14 −172.9916 5.0000 82.56 1.497820 15 −32.2107 0.20001.000000 16 369.6080 6.5000 82.56 1.497820 17 −32.4550 2.0000 25.411.805182 18 −49.6675 d3 1.000000 19 PL 0.0000 1.000000 Fai 6.000012.0000 20.0000 f 100.0000 250.0000 750.0000 D0 0.0000 0.0000 0.0000 d13.4900 41.4700 58.3500 d2 22.2244 17.5798 2.0979 d3 41.5933 8.25796.8598

In Table 4, the surface number 1 corresponds to the aperture stop (SP)12, the surface numbers 2 to 18 are numbers assigned to lens surfaces inorder from the object side, and the surface number 19 corresponds to alens-tube abutting joint surface (PL). The distance between the lenssurface (2) closest to the object side and the aperture stop (SP) is 5mm. f represents focal length of the whole lens system when the distancefrom the object plane to the aperture stop 12 is defined as d₀=∞ andfocal length of an imaging optical system is f=250 mm. Fno represents Fnumber and fai represents the stop diameter of the aperture stop 12.

Values corresponding to the aforesaid conditional expressions (1), (2)are as follows.β2L=−0.221  (1)1/β3L=0.000  (2)Ymax/fL=0.110  (3)

FIG. 13 to FIG. 15 show various aberrations (spherical aberration,astigmatism, distortion aberration, chromatic aberration ofmagnification, and coma aberration) in a zoom microscope including theafocal zoom system 70 and the aperture stop 12 based on the variousoriginal values in Table 4 and the imaging optical system 14 based onthe various original values in Table 3. FIG. 13 corresponds to alow-power end state (f=100), FIG. 14 to a mid-power state (f=250), andFIG. 15 to a high-power end state (f=750). As is apparent from thedrawings, the various aberrations are well corrected in the zoommicroscope including the three-group structure afocal zoom system 70.

MODIFICATION EXAMPLE

Incidentally, the above embodiments have described the examples wherethe stop diameter of the aperture stop 12 is variable according to themovement of the variable-power lens groups (G2, G3), but the presentinvention is not limited thereto. The present invention is alsoapplicable to a case where the variable-power lens groups (G2, G3) aremoved while the stop diameter is kept fixed.

Further, the above embodiments have described the examples where thedistances from the abutting joint surfaces of the respective objectivelenses 11 to the rear focal plane are equal, but the present inventionis not limited thereto. The present invention is also applicable to acase where the distance from the abutting joint surface to the rearfocal plane is different depending on each objective lens 11. In thiscase, telecentricity on the object side can be maintained by moving theaperture stop 12 in the optical axis direction when the objective lens11 is replaced.

Further, in the above-described second embodiment, the coaxialepi-illuminator (43 to 46) is provided between the afocal zoom system 13and the imaging optical system 14, but the present invention is notlimited thereto. The coaxial epi-illuminator (43 to 46) may be providedbetween the objective lens 11 and the afocal zoom system 13. In thiscase, flare and contrast deterioration due to self-fluorescence on thelens planes of the observation optical system can be reduced.

Further, in the above-described third embodiment, the fluorescenceepi-illuminator (51 to 56) is disposed between the objective lens 11 andthe aperture stop 12, but the present invention is not limited thereto.The fluorescence epi-illuminator (51 to 56) may be provided between theaperture stop 12 and the afocal zoom system 13, or may be providedbetween the afocal zoom system 13 and the imaging optical system 14.However, in a case of the fluorescence observation, noise componentsascribable to the self-fluorescence on the lens planes need to bereduced, and therefore, the fluorescence epi-illuminator is preferablyprovided between the objective lens 111 and the afocal zoom system 13.

Fifth Embodiment

A zoom microscope 10 of a fifth embodiment has an objective lens 11, anoptical member 20 for phase contrast observation, an aperture stop 12,an afocal zoom system 13, and an imaging optical system 14, which arearranged in this order from a specimen 10A side, as shown in FIG. 16.Ray bundles emitted from respective points of the specimen 10A areturned to parallel ray bundles via the objective lens 11, are variablymagnified via the afocal zoom system 13, and are collected via theimaging optical system 14 to reach an image plane 10B.

In order to observe an image of the specimen 10A formed on the imageplane 10B, an image sensor, for example, a CCD or the like is disposedon the image plane 10B. Alternatively, instead of the imaging opticalsystem 14, an observation binocular tube (eyepiece tube), a phototube, atrinocular tube for observation and photographing, or the like whichincludes an equivalent imaging optical system can be disposed accordingto an intended use. The use of the zoom microscope 10 of the fifthembodiment enables vertical observation and image acquisition of thespecimen 10A.

Further, in the zoom microscope 10 of the fifth embodiment, the opticalmember 20 for phase contrast observation is provided between theobjective lens 11 and the aperture stop 12. The optical member 20 isinsertable/removable to/from an observation optical path 10C of the zoommicroscope 10. When the optical member 20 is inserted in the observationoptical path 10C, a contrast image of the specimen 10A is formed on theimage plane 10B. The contrast image is a contrasted fine structure ofthe specimen 10A. When the optical member 20 is removed from theobservation optical path 10C, a bright-field image of the specimen 10Ais formed on the image plane 10B.

Therefore, by inserting/removing the optical member 20 to/from theobservation optical path 10C, it is possible to selectively performcontrast observation and bright-field observation of the specimen 10A.Note that the optical member 20 can be disposed at an arbitrary positionbetween the objective lens 11 and the afocal zoom system 13. A desirableposition is near the aperture stop 12.

Further, in the zoom microscope 10 of the fifth embodiment, the afocalzoom system 13 is composed of a first lens group G1 with a positiverefractive power, a second lens group G2 with a negative refractivepower, a third lens group G3 with a positive refractive power, and afourth lens group G4 with a weak positive refractive power, which arearranged in this order from the specimen 10A side, and the second lensgroup G2 and the third lens group G3 are variable-power lens groups.Therefore, by moving the variable-power lens groups (G2, G3) in anoptical axis direction while fixing the first lens group G1 and thefourth lens group G4, it is possible to arbitrarily vary magnificationfor observing the image (the contrast image or the bright-field image)of the specimen 10A. The observation magnification is determined by aratio of a focal length of the objective lens 11 and a focal length ofthe afocal zoom system 13 (combined with the imaging optical system 14).

Further, the objective lens 11 is of an infinity correction type, and arear focal plane of the objective lens 11 is located on an image side ofits lens plane nearest to the image (between the objective lens 111 andthe afocal zoom system 13). The aperture stop 12 is disposed on the rearfocal plane (or near the rear focal plane) of the objective lens 11.Therefore, the position of an object-side entrance pupil of theobjective lens 11 is infinity (telecentric), and principal rays of theray bundles emitted from the respective points of the specimen 10Abecome parallel to the optical axis direction.

Further, setting the position of the aperture stop 12 as the position ofan entrance pupil of the afocal zoom system 13 also makes it possible toset the position of the entrance pupil of the objective lens 11 atinfinity in the whole variable-power range even when the aforesaidvariable-power lens groups (G2, G3) are moved for power variation. Inshort, irrespective of a variable-power state by the afocal zoom system13, telecentricity on the object side of the objective lens 11 can bemaintained.

Moreover, in the zoom microscope 10 of the fifth embodiment, theobjective lens 11 is replaceably attached to a slider or a turret(revolver as shown in FIG. 38). A turret rotation portion is rotatablyattached to a turret fixation portion. Specifically, a plural kinds ofthe objective lenses 11 (for example, a low-power objective lens 11(1)and a high-power objective lens 11(2) shown in FIGS. 17, 38, and so on)different in magnification (different in focal length or numericalaperture) are attached to the turret, and the kind of the objective lens11 can be changed by the rotation (sliding) of the turret.

In the structure where the objective lens 11 is replaced by using theturret, the turret is disposed between the objective lens 11 and theoptical member 20. In order to secure a space for arranging the turret,a distance d from the objective lens 11 to the optical member 20preferably satisfies the following conditional expression (4), where Dis the distance from the objective lens 11 to the afocal zoom system 13.d≧D/2  (4)

Further, in the zoom microscope 10 of the fifth embodiment, thedistances between abutting joint surfaces of the respective objectivelenses 11 (surfaces attached to the turret) and the rear focal plane aresubstantially equal. Therefore, even when the objective lens 11 isreplaced, it is possible to coincide the rear focal plane of theobjective lens 11 (or the vicinity thereof) and an arrangement plane ofthe aperture stop 12 with each other while the aperture stop 12 is keptfixed. It is also possible to maintain the state in which thearrangement plane of the aperture stop 12 and the position of theentrance pupil of the afocal zoom system 13 coincide with each other.

Therefore, even when the objective lens 11 is replaced, telecentricityon the object side of the objective lens 11 can be maintainedirrespective of the variable-power state (i.e., the positions of thevariable-power lens groups G2, G3) of the afocal zoom system 13.

When the low-power objective lens 11(1) is disposed on the optical axisof the afocal zoom system 13, it is possible to change the magnificationfor observing the image (the contrast image or the bright-field image)of the specimen 10A according to a ratio of the focal length of theobjective lens 11(1) and the focal length of the afocal zoom system 13(combined with the imaging optical system 14), while maintainingtelecentricity on the object side of the objective lens 11(1). Likewise,when the high-power objective lens 11(2) is disposed on the optical axisof the afocal zoom system 13, it is possible to change the magnificationfor observing the image (the contrast image or the bright-field image)of the specimen 10A according to a ratio of the focal length of theobjective lens 11(2) and the focal length of the afocal zoom system 13(combined with the imaging optical system 14), while maintainingtelecentricity on the object side of the objective lens 11(2).

A range of the observation magnification (variable-power range) will bedescribed assuming that, for example, the focal length of the low-power(1×) objective lens 11(1) is 100 mm, the focal length of the high-power(4×) objective lens 11(2) is 25 mm, the focal length of a very low-power(0.5×) objective lens (not shown) is 200 mm, and the focal length of theafocal zoom system 13 (combined with the imaging optical system 14) is100 mm to 750 mm. When the very low-power objective lens is used, thevariable-power range is 0.5 to 3.75. When the low-power objective lens11(1) is used, the variable-power range is 1 to 7.5. When the high-powerobjective lens 11(2) is used, the variable-power range is 4 to 30. Theoverall variable-power range is 0.5 to 30.

Thus, in the zoom microscope 10 of the fifth embodiment, the replaceableobjective lenses 11 (see the objective lenses 11(1), 11(2) in FIG. 17)share the afocal zoom system 13 and the variable-power range is shiftedby the replacement of the objective lens 11, so that it is possible toexpand the variable-power range with a simple structure (i.e., only oneafocal zoom system 13).

The use of the very low-power objective lens (for example, 0.5×objective lens) as one of the replaceable objective lenses 11 makes itpossible to expand the variable-power range up to a very lowvariable-power range (about 0.5 to about 2) with a simple structure. Inthis case, the zoom microscope 10 functions as a “macrozoom microscope”,so that macro observation of the specimen 10A is also made possible. Inthe macro observation, a relatively large specimen 10A such as, forexample, a metal specimen and a mechanical part (such as a gear) isobserved. In order to respond to thickness change of the specimen 10A,the whole observation optical system from the objective lens 11 to theimaging optical system 14 is vertically moved.

When the very low-power objective lens (for example, the 0.5× objectivelens) is used and the variable-power lens groups (G2, G3) of the afocalzoom system 13 are moved while the optical member 20 for phase contrastobservation is inserted in the observation optical path 10C of the zoommicroscope 10, it is possible to carry out the observation of thecontrast image of the specimen 10A while zooming it at an arbitrarymagnification in the low variable-power range (in the power range fromabout 0.5 to about 2). Further, the replacement of the objective lensenables the observation of the contrast image of the specimen 10A whilezooming it at an arbitrary magnification in a wide variable-power range(for example, a power range from 0.5 to 30) including the aforesaid lowvariable-power range.

Further, in the zoom microscope 10 of the fifth embodiment, evenreplacing the objective lenses 11 does not necessitate replacing theoptical member 20 for phase contrast observation. The optical member 20is a member common to the plural objective lenses 11, and the kind ofthe objective lens 11 is changeable while the position of the opticalmember 20 is kept fixed. Therefore, only replacing the objective lens 11by the rotation of the turret can change the observation magnificationof the contrast image of the specimen 10A to a magnification in, forexample, the aforesaid range (the power range from 0.5 to 30). Sharingthe optical member 20 makes it possible to configure the zoom microscope10 at low cost.

Further, in the zoom microscope 10 of the fifth embodiment, as has beendescribed, even when the objective lens 11 is replaced, telecentricityon the object side can be ensured irrespective of the variable-powerstate of the afocal zoom system 13 (i.e., the positions of thevariable-power lens groups G2, G3)). Therefore, even when the objectivelens for use is changed to the low-power objective lens and the macroobservation of the specimen 10A is carried out while the variable-powerlens groups (G2, G3) are moved, telecentricity on the object side can besimilarly ensured. Inserting the optical member 20 makes it possible toconstantly obtain a good contrast image free of unevenness in field ofview.

Further, in the microscope 10 of the fifth embodiment, preferably, avariable aperture stop is used as the aperture stop 12, thereby making astop diameter thereof variable according to the movement of thevariable-power lens groups (G2, G3) of the afocal zoom system 13 (seeFIG. 18( a) and FIG. 18( b)). In FIG. 18( a) and FIG. 18( b), out of theray bundles emitted from the respective points of the specimen 10A, acenter ray bundle is shown by the broken lines and peripheral principalrays closest to the image are shown by the two-dot chain lines. Notethat not-shown principal rays as well as the shown principal rays arealso parallel to the optical axis direction, and it is seen thattelecentricity on the object side of the objective lens 11 is ensured.

FIG. 18( a) shows a state in which the lens groups (G2, G3) are moved toa low-power side, and reducing the stop diameter of the aperture stop 12in conjunction with this movement makes it possible to regulate theopening angle of the center ray bundle to a small value. In this case,deep depth of focus observation (high field of view) with low NA becomespossible. FIG. 18( b) shows a state in which the lens groups (G2, G3)are moved to the high-power side, and increasing the stop diameter ofthe aperture stop 12 in conjunction with this movement makes it possibleto widen the opening angle of the center ray bundle. In this case,high-resolution observation with high NA (small field of view) ispossible. Adjusting the stop diameter of the aperture stop 12 makes itpossible to constantly keep the contrast of the contrast image of thespecimen 10A property when the optical member 20 is inserted, so that agood contrast image is obtainable.

Further, in the zoom microscope 10 of the fifth embodiment,telecentricity on the object side of the objective lens 11 can beensured in the whole wide variable-power range (for example, a powerrange from 0.5 to 30), which enables coaxial epi-illumination free ofvignetting.

Further, in the zoom microscope 10 of the fifth embodiment, inserting acoaxial epi-illuminator, a fluorescence epi-illuminator, a phototube, orthe like in front of or at the back of the afocal zoom system 13 (thatis, between the objective lens 111 and the afocal zoom system 13, orbetween the afocal zoom system 13 and the imaging optical system 14) canrealize a diversity of observation methods in a wide variable-powerrange (also in the low variable-power range).

Especially because the objective lens 11 is replaceable, an intervalbetween the objective lens 11 and the afocal zoom system 13 ischangeable. Therefore, in addition to the optical member 20 for phasecontrast observation, it is possible to dispose, for example, afluorescence epi-illuminator or an optical system such as an AF systembetween the objective lens 11 and the afocal zoom system 13, whennecessary. In short, the structure where the objective lens 11 isreplaceable realizes the zoom microscope 10 with high scalability.

Here, if differential interference contrast observation is assumed as anexample of the phase contrast observation of the specimen 10A, abirefringence optical member 21 for differential interference contrastobservation (hereinafter, referred to as a “DIC prism 21”) shown in FIG.19 is used as the optical member 20. The DIC prism 21 is a parallel flatplate formed of two wedge-shaped prisms 2A, 2B joined together, and isfor example, a Wollaston prism, a Nomarski prism, or the like. FIG. 19is an enlarged view showing an area between the objective lens 11 andthe aperture stop 12 shown in FIG. 16. Moving the DIC prism 21 in adirection perpendicular to the observation optical path 10C can change abackground contrast of a differential interference contrast image of thespecimen 10A.

In the case of the DIC prism 21, preferably, an error amount ΔZ (mm) ofthe distance from the abutting joint surface of the replaceableobjective lens 11 to the rear focal plane satisfies the followingconditional expression (5). The error amount ΔZ departing from the rangeof the conditional expression (5) causes problems such as worsening anonuniform field of view asymmetrically and lowering the contrast of thedifferential interference contrast image of the specimen 10A. Moreover,within the range of the conditional expression (5), the followingconditional expression (6) is more preferably satisfied.ΔZ≦3.5 mm  (5)ΔZ<1 mm  (6)

Further, for the differential interference contrast observation of thespecimen 10A, an analyzer 22 is removably inserted between the DIC prism21 and the aperture stop 12. These DIC prism 21 and analyzer 22, whichexhibit effects when making a pair, are both inserted in the observationoptical path 10C at the time of the differential interference contrastobservation of the specimen 10A. Then, at the time of the bright-fieldobservation, both are removed from the observation optical path 10C.

Setting the DIC prism 21 and the analyzer 22 perpendicular to theobservation optical path 10C at the time of the differentialinterference contrast observation of the specimen 10 causes reflectionlight on surfaces thereof to directly enter the image plane 10B, whichwill be a cause of flare. Therefore, preferably, the DIC prism 21 andthe analyzer 22 are set inclined relative to the observation opticalpath 10C. Further, an inclination angle thereof is preferably largerthan an angle of the principal ray relative to an image point with themaximum field number.

Moreover, for the differential interference contrast observation of atransparent specimen 10A, a transilluminator is disposed under thespecimen 10A (opposite the objective lens 11). It is necessary todispose in the transilluminator a DIC prism similar to the aforesaid DICprism 21 and a polarizer that is in cross-Nicol position with respect tothe aforesaid analyzer 22.

In this transilluminator, linear polarized light from the polarizer isdivided via the DIC prism into two ray bundles, which thereafter enterthe specimen 10A. Then, the two ray bundles emitted from the specimen10A interfere with each other via the DIC prism 21 and the analyzer 22of the zoom microscope 10 to turn to a differential interferencecontrast image on the image plane 10B.

Incidentally, in the case of the differential interference contrastobservation, the analyzer 22 is not limited to be positioned between theDIC prism 21 and the aperture stop 12, but may be positioned between theaperture stop 12 and the afocal zoom system 13 (FIG. 16), or maypositioned between the afocal zoom system 13 and the imaging opticalsystem 14. However, it is necessary to keep the cross-Nicol position ofthe analyzer 22 with respect to the polarizer of the transilluminator.

Further, a possible structure as a modification example of thedifferential interference contrast observation of the specimen 10A maybe such that a slit is disposed instead of the DIC prism and thepolarizer of the transilluminator, and the specimen 10A is illuminatedby nonpolarized light from the slit (Japanese Unexamined PatentApplication Publication No. 2003-322798). In this case, a polarizer isdisposed on the objective lens 11 side of the DIC prism 21 of the zoommicroscope 10.

Further, when phase difference observation is assumed as another exampleof the phase contrast observation of the specimen 10A, a phase plate(for example, a phase ring or a phase dot) is used as the optical member20. The phase plate is preferably disposed near the aperture stop 12.Further, the use of a diffraction grating as the optical member 20 makesit possible to obtain a pseudo differential interference contrast image(Japanese Unexamined Patent Application Publication No. Hei 11-95174,Japanese Unexamined Patent Application Publication No. Hei 7-281099, andso on). Further, an ND plate for HMC (Hoffman Modulation Contrast) maybe used as the optical member 20 (Japanese Unexamined Patent ApplicationPublication No. Sho 51-29148, the specification of U.S. Pat. No.4,200,354).

Sixth Embodiment

A zoom microscope 40 of a sixth embodiment is configured such that acoaxial epi-illuminator (41 to 45) is disposed between the afocal zoomsystem 13 and the imaging optical system 14 of the zoom microscope 10 ofthe fifth embodiment (FIG. 16), as shown in FIG. 20. This zoommicroscope 40 is used for differential interference contrast observationof a transparent specimen 10A in industrial application.

In the zoom microscope 40 of the sixth embodiment, a DIC prism 21similar to that in FIG. 19 is used as an optical member for phasecontrast observation and this DIC prism 21 is disposed between theobjective lens 11 and the aperture stop 12, and an analyzer 22 isdisposed between the coaxial epi-illuminator (41 to 45) and the imagingoptical system 14. Further, a polarizer 44 in cross-Nicol position withrespect to the analyzer 22 is disposed in the coaxial epi-illuminator(41 to 45).

In the coaxial epi-illuminator (41 to 45), an outgoing ray bundle from afiber light source 41 is guided to the afocal zoom system 13 via acollector lens 42, a relay lens 43, the polarizer 44, and a beamsplitter 45 to reach the aperture stop 12 via the afocal zoom system 13.At this time, an image of a light source (an end face image of the fiberlight source 41) is formed on the aperture stop 12 (or in its vicinity)by the coaxial epi-illuminator (41 to 45).

Thereafter, the ray bundle that has passed through the aperture stop 12enters the specimen 10A via the DIC prism 21 and the objective lens 11.Thus, in the zoom microscope 40, linear polarized light from thepolarizer 44 is divided via the DIC prism 21 into two ray bundles, whichthereafter enter the specimen 10A. Then, the two ray bundles emittedfrom the specimen 10A interfere with each other via the DIC prism 21 andthe analyzer 22 to turn to a differential interference contrast image onthe image plane 10B.

As described above, since the rear focal plane of the objective lens 11is located near the aperture stop 12 and telecentricity on the objectside of the objective lens 11 can be ensured in the whole widevariable-power range (for example, a power range from 0.5 to 30), aprincipal ray of the ray bundle directed toward the specimen 10A fromthe objective lens 11 becomes parallel to the optical axis direction. Inshort, the illumination for the specimen 10A becomes coaxialepi-illumination (so-called telecentric illumination) free ofvignetting.

This enables good differential interference contrast observation of thetransparent specimen 10A. Especially in micro observation in a low-powerrange (in a power range from about 0.5 to about 2), poor telecentricityon the object side is not desirable since it results in a large angle atwhich principal rays (rays passing through the center of the pupil) in ascreen peripheral portion pass through a pupil plane, thereby causingvignetting of the illumination in a field of view. In the zoommicroscope 40 of this embodiment, telecentricity on the object side canbe ensured also in the low-power range, which enables good macroobservation of the differential interference contrast image by thecoaxial epi-illumination free of vignetting.

Further, in the zoom microscope 40 of this embodiment, the coaxialepi-illuminator (41 to 45) is disposed between the afocal zoom system 13and the imaging optical system 14 to illuminate the specimen 10A via theafocal zoom system 13 (that is, the afocal zoom system 13 is shared bythe illumination system and the observation system), and therefore, itis possible to change an illumination range as well in conjunction witha change in an observation range of the specimen 10A at the time ofpower variation. This enables efficient coaxial epi-illumination anddifferential interference contrast observation of the specimen 10A.

Incidentally, when a diffraction grating is used instead of the DICprism 21 as the optical member for phase contrast observation (JapaneseUnexamined Patent Application Publication No. Hei 11-95174, JapaneseUnexamined Patent Application Publication No. Hei 7-281099, and so on),good differential interference contrast observation similar to the aboveis also possible. In this case, the polarizer 44 and the analyzer 22 arenot provided.

In the above-described sixth embodiment, the coaxial epi-illuminator (41to 45) is provided between the afocal zoom system 13 and the imagingoptical system 14, but the present invention is not limited thereto. Thecoaxial epi-illuminator (41 to 45) may be provided between the objectivelens 11 and the afocal zoom system 13. In this case, it is possible toreduce flare and contrast deterioration ascribable to self-fluorescenceon each lens plane in the observation optical system (from the objectivelens 11 to the imaging optical system 14).

Seventh Embodiment

A zoom microscope 50 of a seventh embodiment is configured such that afluorescence epi-illuminator (51 to 56) is provided between theobjective lens 11 and the aperture stop 12 of the zoom microscope 10 ofthe fifth embodiment (FIG. 16) as shown in FIG. 21. Under a specimen10A, a not-shown transilluminator is further disposed. The zoommicroscope 50 is used for fluorescence observation and differentialinterference contrast observation based on feeble light from thetransparent specimen 10A such as a biospecimen marked by a fluorescentsubstance.

In the zoom microscope 50 of the seventh embodiment, a DIC prism 21similar to that in FIG. 19 is used as an optical member for phasecontrast observation and this DIC prism 21 is disposed between theobjective lens 11 and the fluorescence epi-illuminator (51 to 56), andan analyzer 22 is disposed between the fluorescence epi-illuminator (51to 56) and the aperture stop 12. Further, in the not-showntransilluminator, a DIC prism similar to the DIC prism 21 is disposedand a polarizer in cross-Nicol position with respect to the analyzer 22is disposed.

Similar to the above, in the transilluminator, linear polarized lightfrom the polarizer is divided via the DIC prism into two ray bundles,which thereafter enter the specimen 10A. Then, the two ray bundlesemitted from the specimen 10A interfere with each other via the DICprism 21 and the analyzer 22 of the zoom microscope 50 to turn to adifferential interference contrast image on the image plane 10B.

Meanwhile, in the fluorescence epi-illuminator (51 to 56), an outgoingray bundle from a fiber light source 51 enters an excitation filter 54via a collector lens 52, a relay lens 53, and a not-shown aperture stop.The excitation filter 54 transmits only a ray bundle (excitation light)in a wavelength band necessary for exciting the specimen 10A. Theexcitation light from the excitation filter 54 is led to the objectivelens 11 via a dichroic mirror 55 to enter the specimen 10A via theobjective lens 11.

Fluorescence emitted from the specimen 10A enters the aperture stop 12via the objective lens 11, the dichroic mirror 55, and a barrier filter56 and thereafter reaches an image plane 10B via the aperture stop 12,the afocal zoom system 13, and the imaging optical system 14. Thefluorescence from the specimen 10A is feeble and enters the dichroicmirror 55 together with unnecessary excitation light reflected on thespecimen 10A, but the unnecessary excitation light is cut off when itpasses through the dichroic mirror 55 and the barrier filter 56, so thatonly the feeble fluorescence can be led to the image plane 10B.

Therefore, in the zoom microscope 50 of this embodiment, the two imagesoverlapping each other, namely, the differential interference contrastimage of the specimen 10A by the transillumination and a fluorescenceimage of the specimen 10A by the epi-illumination, can be formed on theimage plane 10 b. Consequently, the parallel execution of thefluorescence observation and the differential interference contrastobservation of the specimen 10A becomes possible in the whole widevariable-power range (for example, a power range from 0.5 to 30).

Further, in this embodiment, an interval between the objective lens 11and the afocal zoom system 13 is freely changeable. Therefore, thefluorescence observation and the phase contrast observation can proceedin parallel, with magnification being arbitrarily set in a widemagnification range.

Further, in the zoom microscope 50 of this embodiment, the excitationlight from the fluorescence epi-illuminator (51 to 56) passes onlythrough the objective lens 11 in the observation optical system (fromthe objective lens 11 to the imaging optical system 14) and passesthrough neither of the afocal zoom system 13 and the imaging opticalsystem 14. This can minimize self-fluorescence that is generated by theexcitation light in each lens element of the observation optical system.As a result, fluorescence observation with good contrast is enabledwhile reducing noise components ascribable to the self-fluorescence.

In the above-described seventh embodiment, the fluorescenceepi-illuminator (51 to 56) is provided between the objective lens 11 andthe aperture stop 12, but the present invention is not limited thereto.The fluorescence epi-illuminator (51 to 56) may be disposed between theaperture stop 12 and the afocal zoom system 13. In short, thefluorescence epi-illuminator (51 to 56) can be disposed at an arbitraryposition between the objective lens 11 and the afocal zoom system 13.

Disposing the fluorescence epi-illuminator (51 to 56) between theobjective lens 11 and the afocal zoom system 13 can reduce the number oflenses through which the epi-illumination (excitation light) passes,compared with that when the fluorescence epi-illuminator (51 to 56) isdisposed between the afocal zoom system 13 and the imaging opticalsystem 14. Therefore, transmittance of the epi-illumination becomeshigher, which makes it possible to obtain a bright fluorescence image.Further, the self-fluorescence in each lens element becomes smaller andthus S/N of the fluorescence image is improved. Moreover, an opticalmaterial (glass material) of each lens element can be selected from awider variety of materials.

In the zoom microscope 50 of this embodiment, the distance from theobjective lens 11 to the aperture stop 12 becomes long and thefluorescence illumination is not telecentric illumination, but thetelecentricity of the illumination is not a significant issue in thefluorescence observation since the fluorescence observation observes thefluorescence from a fluorescent substance in the specimen 10A irradiatedwith the excitation light.

MODIFICATION EXAMPLE

The above fifth embodiment to seventh embodiment have described theexamples where the stop diameter of the aperture stop 12 is variableaccording to the movement of the variable-power lens groups (G2, G3),but the present invention is not limited thereto. The present inventionis also applicable to a case where the variable-power lens groups (G2,G3) are moved while the stop diameter is kept fixed.

Further, the above fifth embodiment to seventh embodiment have describedthe examples where the distances from the abutting joint surfaces of therespective objective lenses 11 to the rear focal plane are equal, butthe present invention is not limited thereto. The present invention isalso applicable to a case where the distance from the abutting jointsurface to the rear focal plane is different depending on each objectivelens 11. In this case, telecentricity on the object side can bemaintained by moving the aperture stop 12 in the optical axis directionwhen the objective lens 11 is replaced.

Further, in the above fifth embodiment to seventh embodiment, themagnification of the objective lens 11 is in the power range from 0.5 to4 (in terms of focal length fobj, a range of fobj=25 mm to 200 mm) andthe magnification of the afocal zoom system is in a power range from 1to 7.5 (in terms of focal length fz, in a range of fz=100 mm to 750 mm),but the present invention is not limited thereto. The present inventionis also applicable to a case where the focal length of the objectivelens 11 is set as fobj=5 mm to 400 mm and the focal length fz is set asfz=50 mm to 1000 mm, thereby setting the overall variable-power range ofthe image of the specimen 10A to 0.125 to 200.

Eighth Embodiment

In low-magnification observation, a real field of view becomes large.Specifically, object height becomes large and accordingly a differencein height between a ray bundle on an optical axis passing through a lensand peripheral ray bundles becomes large, so that pupil aberration andchromatic aberration in addition to normal aberration involved inimaging are far more difficult to correct. As has been previouslydescribed, in the epi-illumination observation and the like, an objectside requires substantially telecentric illumination in order to obtainuniform brightness up to the periphery of a field of view, to eliminateperspective, and to reduce a magnification change of an image due to apositional change of the object.

A basic configuration of an objective lens for solving this ischaracterized in that it includes a first lens group G1 having apositive refractive power as a whole, a second lens group G2 having anegative refractive power as a whole, and a third lens group G3 having apositive refractive power as a whole, which are arranged in this orderfrom the object side, and the first lens group G1 includes ameniscus-shaped single lens having a positive refractive power andhaving a convex surface on the object side or a meniscus-shaped cementedlens having a positive refractive power and having a convex surface onthe object side, and the second lens group G2 includes at least onecemented lens and is substantially telecentric on the object side.

Further, in macro observation, differential interference contrastobservation is often required in order to increase the contrast, but inthis case, in order to obtain an image free of uneven color, it isespecially necessary to sufficiently correct axial chromatic aberrationand spherical aberration of an image-side pupil of the objective lens, achange of the spherical aberration due to color, and the like. In orderto obtain a flat image, it is also necessary to efficiently correctPetzval sum relating to curvature of field.

A preferable configuration of the objective lens to this end ischaracterized in that the second lens group G2 has two meniscus-shapedcemented lenses, and the two meniscus-shaped cemented lenses aredisposed so that concave surfaces thereof face each other.

A configuration giving preferable conditions in view of chromaticaberration correction is characterized in that the meniscus-shapedcemented lens, in the first lens group G1, having the positiverefractive power and having the convex surface on the object side, andthe cemented lens, in the second lens group G2, closest to the objectside are each formed of a positive lens and a negative lens bondedtogether, and satisfy the following conditional expressions.1.1<νdp1/νdn1<3.5  (7)1.1<νdp2/νdp2<3.0  (8)

Note that νdp1 is Abbe number of a glass material of the positive lensforming the meniscus-shaped cemented lens in the first lens group G1.νdn1 is Abbe number of a glass material of the negative lens forming themeniscus-shaped cemented lens in the first lens group G1. νdn2 is Abbenumber of a glass material of the negative lens forming the cementedlens closest to the object side among the cemented lenses included inthe second lens group G2. νdp2 is Abbe number of a glass material of thepositive lens forming the cemented lens closest to the object side amongthe cemented lenses included in the second lens group G2.

The conditional expression (7) gives a ratio of the Abbe number νdp1 andthe Abbe number νdn1 of the glass materials of the positive lens and thenegative lens which form the meniscus-shaped cemented lens having thepositive refractive power and having the convex surface on the objectside, in the first lens group G1. In this cemented lens, the positivelens uses the glass material whose Abbe number is larger than that ofthe glass material of the negative lens, so that the cemented lens hasan achromatizing effect. The conditional expression (8) defines a ratioof the Abbe number νdn2 and the Abbe number νdp2 of the glass materialsof the negative lens and the positive lens which form the cemented lensclosest to the object side in the second lens group G2. In contrast tothe conditional expression (7), the negative lens uses the glassmaterial whose Abbe number is larger than that of the positive lens, sothat a cemented surface has a chromatizing effect.

Thus, satisfying both the conditional expression (7) and the conditionalexpression (8) can realize chromatic aberration correction of both thewhole objective lens system and the exit pupil. The ratio over the upperlimit value of the conditional expression (7) results in excessiveachromatizing of the first lens group, which makes it difficult tocorrect chromatic aberration of magnification. On the other hand, theratio below the lower limit value results in insufficient achromatizingof the first lens group, which increases axial chromatic aberration ofthe image-side pupil of the objective lens. Here, by defining the upperlimit value and the lower limit value of the conditional expression (7)as 2.9 and 1.9 respectively, it becomes possible to correct thechromatic aberration of the whole objective lens system and thechromatic aberration of the exit pupil more efficiently.

Further, a configuration giving preferable conditions for enabling theconnection to a zoom optical system is characterized in that itsatisfies the following conditional expression (9).0.8<EP/OD<1.5  (9)

Note that EP is the distance on the optical axis from the objectposition to the position of the image-side pupil of the objective lens.OD is the distance on the optical axis from the object position to thefarthest plane from the object in the objective lens.

Even the macro observation often requires some degree of magnifiedobservation, and thus a zoom function is also often demanded. At thistime, in order to connect the objective lens to the zoom optical systemsubsequent to the objective lens without causing any vignetting of theray bundle, it is necessary to put the exit pupil of the objective lenstoward the image side and preferably, to position it outside the lens.Even when such a configuration is adopted, it is necessary to wellcorrect axial chromatic aberration of the pupil, spherical aberration ofthe pupil, and off-axis aberration such as astigmatism and comaaberration of the whole objective lens system. The conditionalexpression (9) is apreferable condition for adopting the aforesaidconfiguration. The ratio over the upper limit value of the conditionalexpression (9) results in an excessively large distance of the exitpupil away from the object plane, which makes it difficult to correctthe off-axis aberrations such as astigmatism and coma aberration of theobjective lens, and also makes the correction in the subsequent opticalsystem impossible, and consequently, the configuration of the wholeoptical system becomes difficult. On the other hand, the ratio under thelower limit value of the conditional expression (9) results in anexcessively short distance of the image-side pupil from the objectplane, which makes it difficult to correct the axial chromaticaberration of the pupil and the spherical aberration of the pupil and inaddition, makes the connection to the subsequent zoom optical systemdifficult. Setting the upper limit value and the lower limit value ofthe conditional expression (9) to 1.2 and 0.9 respectively makes itpossible to correct the aforesaid aberrations more efficiently.

Further, a configuration giving a conditional expression that defines apreferable degree of reduction in deviation from complete telecentricityis characterized in that it satisfies the following conditionalexpression (10).−0.3<α<0.3  (10)

Note that α is an angle (in a degree unit) of the principal ray relativeto the optical axis at a given position on the object.

α falling outside this range of the conditional expression (10) causesuneven illumination at the time of the epi-illumination, so that animage with uniform brightness is not obtainable. α exceeding the upperlimit value of the conditional expression (10) results in an excessivelylarge lens diameter, which is problematic in view of a systemconfiguration.

The following expressions are conditional expressions that arepreferably further satisfied in view of the system configuration.0.5<|R1/f2|<1.5  (11)0.5<|R2/f2|<1.8  (12)

Note that R1 is a radius of curvature of the concave surface of theobject-side cemented lens, out of the meniscus-shaped cemented lenseshaving the concave surfaces facing each other in the second lens groupG2. R2 is a radius of curvature of the concave surface facing theconcave surface with the aforesaid R1, in the image-side cemented lensout of the meniscus-shaped cemented lenses having the concave surfacesfacing each other in the second lens group G2. f2 is focal length of thesecond lens group G2.

The conditional expression (11) and the conditional expression (12)relate to the correction of coma aberration and Petzval sum of the wholeobjective lens system. The ratios exceeding the upper limit values ofthe conditional expression (11) and the conditional expression (12) bothresult in excessively large radii of curvature of the concave surfacesfacing each other, which makes the correction of the Petzval sumdifficult, so that the curvature of field remains. On the other hand,the ratios under the lower limit values result in excessively smallradii of curvature of the concave surfaces facing each other, whichmakes the correction of the coma aberration, astigmatism, and so ondifficult.

The following expression is another conditional expression furtherpreferably satisfied in view of a configuration of the objective lens.0.5<|HR3/Y|<1.1  (13)

Note that HR3 is the height of the principal ray corresponding to themaximum object height in the cemented surface of the meniscus-shapedcemented lens having the positive refractive power and having the convexsurface on the object side, which is included in the first lens groupG1. Y is the maximum object height.

The conditional expression (13) relates to the correction of thelongitudinal chromatic aberration and the ensuring of telecentricity.The ratio over the upper limit value of the conditional expression (13)results in an excessively high position of the principal ray in thiscemented surface compared with the object height, which makes itdifficult to correct the chromatic aberration of magnification in thewhole objective lens system and results in a large lens diameter, sothat it becomes difficult to ensure telecentricity. The ratio below thelower limit value results in an excessively low position of theprincipal ray in this cemented surface compared with the object height,which makes it difficult to correct the longitudinal chromaticaberration of the pupil and spherical aberration of the pupil on theexit pupil.

FIG. 22 is a view showing a configuration of the objective lens of theeighth embodiment according to the present invention. Note that FIG. 22also shows optical paths of rays emitted from four points, on theobject, in an area from the center up to the outermost periphery of theobject. The objective lens of the eighth embodiment is composed of afirst lens group G1, a second lens group G2, and a third lens group G3which are arranged in this order from the object side. The first lensgroup G1 is composed of: a first lens component L11 being a biconvexlens whose surface on the object side has a larger refractive power; anda second lens component ML1 which is a cemented meniscus lens having apositive refractive power and having a convex surface on the object sideand which is composed of a lens L12 and a lens L13. The second lensgroup G2 is composed of two meniscus-shaped cemented lenses, namely, athird lens component ML2 composed of a lens L21 and a lens L22, and afourth lens component ML3 composed of a lens L23 and a lens L24. ML2 andML3 are arranged so that concave surfaces thereof face each other. Thethird lens group G3 is composed of a fifth lens component L31 being abiconvex lens whose surface on the image side has a larger refractivepower.

This embodiment is an infinity objective lens designed such that totalmagnification β=−1 and NA=0.03 when the minimum focal length of thesubsequent zoom lens system is 100 mm, and a zoom ratio can be increasedto a maximum of about 8× and NA can be increased to a maximum of 0.1.

The following Table 5 shows various original values of the eighthembodiment. In Table 5, β is an adaptable magnification range, NA isnumerical aperture on the object side, F is synthesized focal length ofthe whole objective lens system (in mm unit), and D0 is the distance (inmm unit) from the object to the first lens plane. The numerals on theleft end represent the order from the object side, R represents a radiusof curvature (in mm unit) of each lens plane, D represents lensthickness and each interval between lens planes (in mm unit), ndrepresents refractive power for the d-line (λ=587.562 nm), and νdrepresents the Abbe number of the glass material of each lens.

TABLE 5 β = −1~−7.5, NA = 0.03~0.1, DO = 37.8, F = 99.7 NO. R D nd νd 1:39.25410 5.500000 1.59240 68.3 2: −133.28791 0.500000 3: 27.697376.718011 1.59240 68.3 4: −64.55481 2.000000 1.72047 34.7 5: 43.447549.949972 6: 20.55359 4.717811 1.71736 29.5 7: −28.90048 2.000000 1.8348142.7 8: 10.74737 12.641442 9: −11.15870 5.955076 1.83481 42.7 10: 121.28201 5.518289 1.49782 82.5 11:  −16.47334 0.500000 12:  1825.607195.500000 1.59240 68.3 13:  −21.22923

FIG. 23 to FIG. 25 are views showing various aberrations in the eighthembodiment.

FIG. 23 is an aberration view showing spherical aberration, astigmatism,distortion aberration, and coma aberration which are calculated as anideal lens when focal length of a zoom optical system (including animaging lens) subsequent to the objective lens is 100 mm. As shown inFIG. 23, in the objective lens of the eighth embodiment, good correctionis made in respective wavelengths of the d-line (587.562 nm), C-line(656.273 nm), F-line (486.133 nm), and g-line (435.835 nm), with NA=0.03and field number=22.

FIG. 24 is an aberration view showing the aforesaid aberrationscalculated as an ideal lens when the focal length of the zoom opticalsystem (including the imaging lens) subsequent to the objective lens is400 mm. As shown in FIG. 24, in the objective lens of the eighthembodiment, good correction is made in the respective wavelengths of thed-line, C-line, F-line, and g-line, with NA=0.06 and field number=2.

FIG. 25 is an aberration view showing the aforesaid aberrationscalculated as an ideal lens when the focal length of the zoom opticalsystem (including the imaging lens) subsequent to the objective lens is750 mm. As shown in FIG. 25, in the objective lens of the eighthembodiment, good correction is made in the respective wavelengths of thed-line, C-line, F-line, and the g-line, with NA=0.1 and field number=22.

Ninth Embodiment

FIG. 26 is a configuration view (optical path view) of an objective lensof a ninth embodiment according to the present invention. The ninthembodiment has the same configuration as that of the above-describedeighth embodiment except that L12 constituting a first lens group G1 isa meniscus-shaped single lens with a positive refractive power. Theninth embodiment is an infinity objective lens designed such that totalmagnification β=−1 and NA=0.03 when the minimum focal length of asubsequent zoom lens system is 100 mm, and a zoom ratio can be increasedto a maximum of about 8× and NA can be increased to a maximum of 0.1.

The following Table 6 shows various original values of the ninthembodiment as in the above-described eighth embodiment.

TABLE 6 β = −1~−7.5, NA = 0.03~0.1, DO = 38.0, F = 99.6 NO. R D nd νd 1:44.99800 5.500000 1.59240 68.3 2: −154.71000 0.500000 3: 25.231005.000000 1.59240 68.3 4: 53.22200 6.500000 5: 25.47300 7.500000 1.4978282.5 6: −40.27000 3.000000 1.75500 52.3 7: 11.27600 15.250000 8:−12.87300 6.000000 1.80400 46.6 9: 59.78600 5.500000 1.49782 82.5 10: −18.89500 1.000000 11:  1128.31000 5.500000 1.59240 68.3 12:  −21.60030

FIG. 27 to FIG. 29 show various aberrations in the ninth embodiment.

FIG. 27 is a view showing various aberrations, as in the eighthembodiment, which are calculated as an ideal lens when focal length of azoom optical system (including an imaging lens) subsequent to theobjective lens is 100 mm. As shown in FIG. 27, in the objective lens ofthe ninth embodiment, good correction is made in respective wavelengthsof the d-line, C-line, F-line, and g-line, with NA=0.03 and fieldnumber=22.

FIG. 28 is a view showing various aberrations calculated as an ideallens when the focal length of the zoom optical system (including theimaging lens) subsequent to the objective lens is 400 mm.

As shown in FIG. 28, in the objective lens of the ninth embodiment, goodcorrection is made in the respective wavelengths of the d-line, C-line,F-line and g-lin, with NA=0.06 and field number=22.

FIG. 29 is a view showing various aberrations calculated as an ideallens when the focal length of the zoom optical system (including theimaging lens) subsequent to the objective lens is 750 mm.

As shown in FIG. 29, in the objective lens of the ninth embodiment, goodcorrection is made in the respective wavelengths of the d-line, C-line,F-line, and g-line, with NA=0.1 and field number=22.

Tenth Embodiment

FIG. 30 is a configuration view (optical path view) of an objective lensof a tenth embodiment according to the present invention. The tenthembodiment is composed of a first lens group G1, a second lens group G2,and a third lens group G3 which are arranged in this order from anobject side. The first lens group G1 is composed of: a first lenscomponent L11 being a biconvex lens; and a second lens component ML1which is a cemented meniscus lens having a positive refractive power andhaving a convex surface on the object side and which is composed of alens L12 and a lens L13. The second lens group G2 is composed of: athird lens component L21 being a meniscus lens having a concave surfaceon an image side; and two meniscus-shaped cemented lenses, namely, afourth lens component ML2 composed of a lens L22 and a lens L23 and afifth lens component ML3 composed of a lens L24 and a lens L25. ML2 andML3 are arranged so that concave surfaces thereof face each other. Thethird lens group G3 is composed of a sixth lens component L31 being abiconvex lens whose image-side surface has a larger refractive power.The tenth embodiment is an infinity objective lens designed such thattotal magnification β=−0.5 and NA=0.015 when the minimum focal length ofa subsequent zoom lens system is 100 mm, and a zoom ratio can beincreased to a maximum of about 8× and NA can be increased to a maximumof 0.05.

The following Table 7 shows various original values of the tenthembodiment as in the above-described eighth embodiment.

TABLE 7 β = −0.5~−3.75, NA = 0.015~0.05, DO = 52.0, F = 198.0 NO. R D ndνd 1: 74.30554 6.500000 1.77250 49.6 2: 2037.03211 0.500000 3: 36.3959910.400000 1.75500 52.3 4: INFINITY 3.000000 1.80809 22.8 5: 45.763494.000000 6: 22.64802 4.000000 1.64000 60.1 7: 16.06360 22.600000 8:28.01779 5.000000 1.80809 22.8 9: −28.01779 2.000000 1.80400 46.6 10: 12.31254 13.950000 11:  −17.06184 2.000000 1.90265 35.7 12:  27.416845.500000 1.49782 82.5 13:  −17.14338 7.000000 14:  −130.69481 6.0000001.59240 68.3 15:  −19.30158

FIG. 31 to FIG. 33 show various aberrations in the tenth embodiment.

FIG. 31 is a view showing various aberrations, as in the eighthembodiment, which are calculated as an ideal lens when focal length of azoom optical system (including an imaging lens) subsequent to theobjective lens is 100 mm. As shown in FIG. 31, in the objective lens ofthe tenth embodiment, good correction is made in respective wavelengthsof the d-line, C-line, F-line, and g-line, with NA=0.015 and fieldnumber=22.

FIG. 32 is a view showing various aberrations calculated as an ideallens when the focal length of the zoom optical system (including theimaging lens) subsequent to the objective lens is 400 mm. As shown inFIG. 32, in the objective lens of the tenth embodiment, good correctionis made in the respective wavelengths of the d-line, C-line, F-line, andg-line, with NA=0.03 and the field number=22.

FIG. 33 is a view showing various aberrations calculated as an ideallens when the focal length of the zoom optical system (including theimaging lens) subsequent to the objective lens is 750 mm. As shown inFIG. 33, in the objective lens of the tenth embodiment, good correctionis made in the respective wavelengths of the d-line, C-line, F-line, andg-line, with NA=0.05 and field number=22.

Eleventh Embodiment

FIG. 34 is a configuration view (optical path view) of an objective lensof an eleventh embodiment according to the present invention. Theeleventh embodiment has a similar configuration as that of theabove-described tenth embodiment, and is an infinity objective lensdesigned such that total magnification β=−0.5 and NA=0.015 when focallength of a subsequent zoom lens system is 100 mm, and a zoom ratio canbe increased to a maximum of about 8× and NA can be increased to amaximum of 0.05.

The following Table 8 shows various original values of the eleventhembodiment, as in the above-described eighth embodiment.

TABLE 8 β = −0.5~−3.75, NA = 0.015~0.05, DO = 52.0, F = 198.0 NO. R D ndνd 1: 171.62745 5.882876 1.77250 49.6 2: −171.62745 0.500000 3: 35.3503210.656757 1.60300 65.5 4: −968.99678 3.000000 1.80809 22.8 5: 57.211740.500000 6: 21.16891 5.566517 1.72916 54.7 7: 17.03188 21.998566 8:31.69094 5.000000 1.80809 22.8 9: −31.69094 2.000000 1.81600 46.6 10: 12.10648 20.297055 11:  −16.97407 2.000000 1.88300 40.8 12:  43.257105.713904 1.49782 82.5 13:  −18.51337 3.378728 14:  −412.62298 5.9555971.59240 68.3 15:  −20.57021

FIG. 35 to FIG. 37 show various aberrations in the eleventh embodiment.

FIG. 35 is a view showing various aberrations, as in the eighthembodiment, which are calculated as an ideal lens when focal length of azoom optical system (including an imaging lens) subsequent to theobjective lens is 100 mm. As shown in FIG. 35, in the objective lens ofthe eleventh embodiment, good correction is made in respectivewavelengths of the d-line, C-line, F-line, and g-line, with NA=0.015 andfield number=22.

FIG. 36 is a view showing various aberrations calculated as an ideallens when the focal length of the zoom optical system (including theimaging lens) subsequent to the objective lens is 400 mm. As shown inFIG. 36, in the objective lens of the eleventh embodiment, goodcorrection is made in respective wavelengths of the d-line, C-line,F-line, and g-line, with NA=0.03 and field number=22.

FIG. 37 is a view showing various aberrations calculated as an ideallens when the focal length of the zoom optical system (including theimaging lens) subsequent to the objective lens is 750 mm. As shown inFIG. 37, in the objective lens of the eleventh embodiment, goodcorrection is made in the respective wavelengths of the d-line, C-line,F-line, and g-line, with NA=0.05 and field number=22.

Corresponding values in the respective embodiments will be shown below.

TABLE 9 Embodiment NO. 8 NO. 9 NO. 10 NO. 11 νdp1/νdn1 1.97 — 2.29 2.87νdn2/νdp2 1.45 — 2.04 2.04 EP/OD 1.00 1.00 1.07 1.12 maximum object0.057 0.057 0.023 −0.264 height α f2 −13.97 −11.81 −11.25 −12.56 |R1/f2|0.769 0.955 1.094 0.964 |R2/f2| 0.799 1.090 1.517 1.351 |HR3/Y| 0.80 —0.832 0.832

The embodiments hitherto have described, as examples, the very low-powerobjective lens whose unsigned value of the total magnification is equalto or smaller than 1×, when the minimum focal length of the subsequentzoom lens system is 100 mm (end of a wide field of view), but it goeswithout saying that the present invention is applicable to a low-powerrange objective lens.

The invention is not limited to the above embodiments and variousmodifications may be made without departing from the spirit and scope ofthe invention. Any improvement may be made in part or all of thecomponents.

1. A zoom microscope comprising a replaceable infinity correctionobjective lens, an aperture stop, an afocal zoom system, and an imagingoptical system which are arranged in this order, seen from a specimenside, wherein said aperture stop is disposed on or near a rear focalplane of said objective lens and at a side closer to said objective lensthan all lenses of the afocal zoom system, and wherein the afocal zoomsystem includes, in order, from the specimen side: a first lens grouphaving a positive refractive power; a second lens group having anegative refractive power; a third lens group having a positiverefractive power, and a fourth lens group having a positive refractivepower, wherein the afocal zoom system varies by moving the second lensgroup and the third lens group in an optical axis direction, and whereina magnification β2L of the second lens group is in a low-power endstate, a magnification β3L of the third lens group is in the low-powerend state, and a focal length fL of the optical system which is acombination of said afocal zoom system and said imaging optical systemin the low-power state, and a maximum image height Y max in thelower-power end state satisfy following conditional expressions:−0.1<β2L<−0.3−0.01<1/β3L<0.040.05<Ymax/fL<0.16.
 2. The zoom microscope according to claim 1, furthercomprising a holding member that replaces the objective lens withanother objective lens while holding a plurality of objective lensesincluding the objective lens, wherein said aperture stop is disposed onan image side of said holding member.
 3. The zoom microscope accordingto claim 1, further comprising an optical member for phase contrastobservation that is removably inserted between said objective lens andsaid afocal zoom system.
 4. The zoom microscope according to claim 3,further comprising a fluorescence epi-illuminator provided between saidobjective lens and said afocal zoom system.
 5. The zoom microscopeaccording to claim 3, wherein: said objective lens is plural in numberand the plural objective lenses are different in magnification from eachother; and distances from abutting joint surfaces of said pluralobjective lenses to said rear focal plane are substantially same.
 6. Thezoom microscope according to claim 5, wherein said optical member forphase contrast observation is common to said plural objective lenses. 7.The zoom microscope according to claim 3, wherein said optical memberfor phase contrast observation is a birefringence optical member fordifferential interference contrast observation.
 8. The zoom microscopeaccording to claim 1, wherein: the afocal zoom system has avariable-power lens group movable in the optical axis direction; and theaperture stop has a stop diameter that is variable in accordance withmovement of the variable-power lens group.
 9. The zoom microscopeaccording to claim 8, further comprising: a coaxial epi-illuminatorprovided between the afocal zoom system and the imaging optical systemto form an image of a light source on or near the aperture stop.